Phased array antenna systems can provide rapid beam steering, the ability to generate simultaneous beams, dynamic adjustment of the characteristics of the beam pattern, and graceful degradation.
The underlying principle for beam steering in phased arrays is the alignment of the elemental signals in space such that they add constructively in a desired direction. A direct method of signal alignment would be to use time delay devices, but practical considerations often preclude their use. An often-used alternative is phase based beam steering, in which the elemental signals are appropriately phase shifted. Effectively, a phase gradient (or taper) is applied across the elements. For a given pointing angle, the value of the gradient depends on the signal frequency. Although the frequency dependence can usually be ignored for narrowband applications, this is not true for wideband applications. Failure to compensate for the frequency dependence results in beam pointing errors or beam squint. In a phase based steering approach, timeliness of such compensation is the most important factor in beam pointing accuracy.
Phased array antennas can be used in transmit applications. Applications are progressively moving towards wideband operation. These modern systems may also require accurate, rapid, and dynamic beam forming and steering of modulated radio frequency (RF) signals. Beam forming and steering requirements may demand both sequential beam repositioning and simultaneous directionally independent multiple beams. The ultimate objective is to optimize the trade between own system effective isotropic radiated power (EIRP) and interference to other systems.
Beam forming (e.g., sidelobe control or beam spoiling) may be useful in optimizing intended performance. Beam forming is accomplished by applying a set of amplitude and/or phase weights across the array elements. The accuracy of these weights directly influences the quality of the resulting pattern. In cases where it is important to minimize emissions from the sidelobes (e.g., reducing interference), it is necessary to employ a mechanism for sidelobe control. This is accomplished by using amplitude weighting (i.e., taper) and/or phase adjustments across the elements. The control devices need sufficient resolution in order to achieve the desired beam shaping and level of sidelobe reduction. The quality of sidelobe reduction is directly related to the accuracy of the applied weight values and/or phase adjustments.
In cases where multiple beams are required, an option would be to generate time-multiplexed responses (i.e., sequentially steering to each direction, one at a time). It is imperative that the phase and amplitude control device speed be commensurate with the application. For example in airborne systems, because of the flight dynamics, the beam pointing will need adjustment to keep the beam properly positioned. Support of such beam control changes requires the ability to update the steering controls on the order of hundreds of milliseconds to seconds. For more demanding applications, steering changes may reach on the order of hundreds of nanoseconds. When dealing with multiple beams, another approach is to generate the beams simultaneously. Thus, the beam forming and steering architecture needs additional flexibility to control the individual elements.
Certain architectural implementations of phased array systems inherently possess critical disadvantages such as limited bandwidth, limited accuracy in shaping or pointing the beam, and reduction of effective isotropic radiated power (EIRP) due to inefficient multiple beam generation techniques.
An equation relating frequency (f) and pointing angle (θ) for phase based steering systems is:fH×sin(θH)=fL×sin(θL)where fH represents the highest frequency of the bandwidth and fL is the lowest. Study of this equation reveals that the beam squint, θL−θH, increases with fractional bandwidth, (fH−fL)/fc, where fc represents the center frequency of the bandwidth. Beam squint also increases as scan angle increases. In practice, the wideband limit of any particular architecture is driven by the amount of beam squint that is acceptable for a given application.
For various fractional bandwidths and scan angles, the squinted beam just enters nonvisible space. This leads to no radiation, which theoretically represents an infinite voltage standing wave ratio (VSWR) that will lead to total reflection of output power back into the power amplifier. This could lead to an amplifier damage condition. For the typical values of fractional bandwidth and scan angle encountered in certain applications, the problem of beam squint would be a common occurrence.
In summary, the use of a constant phase in a phase based steering approach is inadequate to steer a wideband signal. There is a need for a method that allows a more complete realization of the full potential of phased array transmit systems, while minimizing the drawbacks.